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r6rs-lib
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<a name="node_chap_16"></a>
<h1 class=chapter>
<div class=chapterheading><a href="r6rs-lib-Z-H-1.html#node_toc_node_chap_16">Chapter 16</a></div><br>
<a href="r6rs-lib-Z-H-1.html#node_toc_node_chap_16"><tt>eval</tt></a></h1>
<p></p>
<p>
The <tt>(rnrs eval (6))</tt><a name="node_idx_1265.5"></a> library allows a program to create Scheme
expressions as data at run time and evaluate them.</p>
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<p></p>
<div align=left><tt>(<a name="node_idx_1266"></a>eval<i> expression environment</i>)</tt>&nbsp;&nbsp;&nbsp;&nbsp;procedure&nbsp;</div>
<p>
Evaluates <i>expression</i> in the specified environment and returns its value.
<i>Expression</i> must be a syntactically valid Scheme expression represented as a
datum value, and <i>environment</i> must be an
<a name="node_idx_1268"></a><em>environment</em>, which can be created using the <tt>environment</tt> procedure described below.</p>
<p>
If the first argument to <tt>eval</tt> is determined not to be a syntactically correct
expression, then <tt>eval</tt> must raise an exception with condition
type <tt>&amp;syntax</tt>.  Specifically, if the first argument to <tt>eval</tt> is a definition or a splicing <tt>begin</tt> form containing a
definition, it must raise an exception with condition type <tt>&amp;syntax</tt>.
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<p></p>
<div align=left><tt>(<a name="node_idx_1270"></a>environment<i> import-spec <tt>...</tt></i>)</tt>&nbsp;&nbsp;&nbsp;&nbsp;procedure&nbsp;</div>
<p>
<i>Import-spec</i> must be a datum representing an
&lt;import spec&gt; (see report
section&nbsp;on &#8220;Library form&#8221;).
The <tt>environment</tt> procedure returns an environment corresponding
to <i>import-spec</i>.</p>
<p>
The bindings of the environment represented by the specifier are
immutable: If <tt>eval</tt> is applied to an expression that is
determined to contain an
assignment to one of the variables of the environment, then <tt>eval</tt> must
raise an exception with a condition type <tt>&amp;assertion</tt>.</p>
<p>
</p>

<tt>(library&nbsp;(foo)<br>
&nbsp;&nbsp;(export)<br>
&nbsp;&nbsp;(import&nbsp;(rnrs))<br>
&nbsp;&nbsp;(write<br>
&nbsp;&nbsp;&nbsp;&nbsp;(eval&nbsp;&#8217;(let&nbsp;((x&nbsp;3))&nbsp;x)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(environment&nbsp;&#8217;(rnrs)))))&nbsp;<br>
&nbsp;<i>&nbsp;writes</i>&nbsp;3<br>
<br>
(library&nbsp;(foo)<br>
&nbsp;&nbsp;(export)<br>
&nbsp;&nbsp;(import&nbsp;(rnrs))<br>
&nbsp;&nbsp;(write<br>
&nbsp;&nbsp;&nbsp;&nbsp;(eval<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#8217;(eval:car&nbsp;(eval:cons&nbsp;2&nbsp;4))<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(environment<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&#8217;(prefix&nbsp;(only&nbsp;(rnrs)&nbsp;car&nbsp;cdr&nbsp;cons&nbsp;null?)<br>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;eval:)))))&nbsp;<br>
&nbsp;<i>&nbsp;writes</i>&nbsp;2<br>
<p></tt>
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